Many different fuel systems are utilized to introduce fuel into the combustion chambers of an engine. One type of fuel system is known as the common rail system. A typical common rail fuel system utilizes one or more pumping mechanisms to pressurize fuel and direct the pressurized fuel to a common manifold, also known as a common rail, which provides a source of pressurised fuel. A plurality of fuel injectors draw pressurized fuel from the common rail and inject one or more shots of fuel per cycle into the combustion chambers. In order to optimize engine operation, fuel within the rail is maintained within a desired pressure range through the precise control of the pumping mechanisms.
Situations may arise in which this precise control is interrupted, pressure fluctuations or spikes occur, or various portions of the fuel system fail. In these situations, there is a possibility that fuel pressures within the common rail could reach levels that have the potential to damage the components of the fuel system. One way to protect the common rail from such excessive pressures is to selectively drain fuel from the common rail as the pressure of the fuel within it exceeds a predetermined maximum threshold value. However, if too much fuel is drained, the pressure of the fuel within the common rail may drop below a certain minimum pressure (at which the fuel injectors and engine will be able to continue operating in at least a limited operational mode, or “limp home” mode) and the engine may shut off. If the engine shuts off suddenly the machine, truck, or other piece of equipment powered by the engine may be left in an undesirable state, position, or location. Moreover, depending on the problem or problems that lead to the excessive pressure within the fuel system, the rate at which the fuel will need to be drained from the common rail to maintain a required minimum pressure may vary.
The incorporation of a pressure relief (or limiter) valve into such a fuel system helps to mitigate, reduce, or even eliminate the adverse effects of excessive fluid pressure on the common rail. When the pressure of the fluid within the system exceeds a maximum threshold value, the pressure relief valve opens and allows fluid to drain from the common rail, thereby lowering the pressure of the fluid within the common rail. The pressure of the fluid may be lowered just enough to protect the common rail without creating instability or completely disabling the system. This means that the engine can still operate.
US-A-2011/0094476 describes a fuel supply system incorporating a pressure relief valve. The pressure relief valve comprises a movable valve member and a resilient member housed in a body which has a fluid inlet and two fluid outlets. The resilient member biases the valve member into a first (closed) position in which the fluid inlet is fluidly blocked from the first outlet and the second outlet. The valve also has a second (open) position in which the inlet is fluidly coupled to the first outlet, but not to the second outlet and a third (open) position in which the inlet is fluidly coupled to both the first and the second outlets, which allows a greater flow of fluid out of the common rail than the second position.
It has been determined that the detection of the opening of the pressure relief valve is useful to aid troubleshooting of low fuel rail pressure problems. It may also be used to trigger a process to attempt to close the pressure relief valve to enable continued use of the machine driven by the engine. It is also recognised that there are some transient conditions which cause the pressure relief valve to open, such as air ingress during a filter change, which cause a rail pressure overshoot which can be remedied quickly, in which case it is desirable to shorten the time taken to reseat the valve and thereby reduce the impact on the machine operator as a result of the engine suddenly losing power.